Lower bound of moving target estimation, in a multi-antenna channel Ultra Wide Band (UWB) Synthetic Aperture Radar (SAR) system is studied in this work. The main focus is to derive lower bound for systems having a very long integration time associated with position and velocity estimation, which is often connected to low frequency systems. Assuming a flat Earth model we consider the lower bound of the speed components in range and in the cross range direction. The estimation in cross range direction is associated with a considerable integration angle, and therefore the lower bound is determined in connection to a SAR process. It is found that the relative speed influences the covariance matrix of the clutter and therefore also the speed estimate. The influence increases the estimation performance of the relative speed and therefore also the performance of the cross range speed estimation.